Don't assume make is gmake

@@ -16,40 +16,40 @@
 DIRS=$(wildcard */)
 SRCS=$(wildcard *.c)
 OBJS=$(SRCS:.c=.o)
 
 all: bin
-        make -C tests
+        $(MAKE) -C tests
 
 include s2n.mk
 
 libs:
-        make -C utils
-        make -C error
-        make -C stuffer
-        make -C crypto
-        make -C tls
-        make -C lib
+        $(MAKE) -C utils
+        $(MAKE) -C error
+        $(MAKE) -C stuffer
+        $(MAKE) -C crypto
+        $(MAKE) -C tls
+        $(MAKE) -C lib
 
 bin: libs
-        make -C bin
+        $(MAKE) -C bin
 
 indent:
-        make -C tests indentsource
-        make -C stuffer indentsource
-        make -C crypto indentsource
-        make -C utils indentsource
-        make -C error indentsource
-        make -C tls indentsource
-        make -C bin indentsource
+        $(MAKE) -C tests indentsource
+        $(MAKE) -C stuffer indentsource
+        $(MAKE) -C crypto indentsource
+        $(MAKE) -C utils indentsource
+        $(MAKE) -C error indentsource
+        $(MAKE) -C tls indentsource
+        $(MAKE) -C bin indentsource
 
 pre_commit_check: all indent clean
 
 clean:
-        make -C tests clean
-        make -C stuffer decruft
-        make -C crypto decruft
-        make -C utils decruft
-        make -C error decruft
-        make -C tls decruft
-        make -C bin decruft
-        make -C lib decruft
+        $(MAKE) -C tests clean
+        $(MAKE) -C stuffer decruft
+        $(MAKE) -C crypto decruft
+        $(MAKE) -C utils decruft
+        $(MAKE) -C error decruft
+        $(MAKE) -C tls decruft
+        $(MAKE) -C bin decruft
+        $(MAKE) -C lib decruft